Usually, in order to protect the semiconductor chip from moisture and ambient pollution the semiconductor devices is sealed with a plastic, ceramic, or a molding resin. In consideration of mass productibility, cost, and other factors, the packaging using the transfer mold forming method is most widely adopted at the present time. In the transfer mold forming method, the semiconductor chip assembly is placed in the mold die, and a thermosetting resin is flowed into the die. The die is brought to a high temperature to cure the resin, forming a package with the chip sealed therein.
There are several intrinsic problems with the transfer mold forming method that must be taken into consideration when the method is used. One problem is related to the warping of the molded semiconductor package. In the case of mold forming, curing is carried out at a temperature as high as 175.degree. C., and the package is then cooled naturally to room temperature. In this case, warping occurs due to the difference between the linear expansion coefficient of silicon as the raw material of the semiconductor chip and the phenol based, epoxy-based or other thermosetting resin used as the molding resin. The warping leads to uneven contact of the connecting terminals on the substrate when the semiconductor package is assembled on a printed-circuit board. As a result, the assembly reliability deteriorates. Also, the larger the package or the chip, the greater the adverse influence of the warping of the package relative to its flatness. As a result, it becomes difficult to effect a highly reliable assembly process.
A method used to solve this problem is forming the molding resin on both sides of the semiconductor chip to the same thickness, that is, centering the semiconductor chip with respect to the package. However, this method cannot be used for certain types of packages. One of these packages is the BGA (Ball Grid Array) package. In the BGA package, the semiconductor chip is attached to an insulating substrate that has solder bumps in a two-dimensional arrangement, and the semiconductor chip is covered. On the insulating substrate, that is, on the surface on the side opposite to the solder bump forming surface of the insulating substrate, the molding resin is used to form the outer shape of the semiconductor device. In this case, the difference between the linear expansion coefficient of the molding resin and the insulating substrate leads to warping. Another semiconductor device that has a structure that does not allow the semiconductor chip to be centered relative to the package is the TQFP (Thin Quad Flat Package), which has an exposed die pad for heat dissipation purposes.
In order to minimize the warping of the package of this type of semiconductor device, it is effective to reduce the thickness of the molding resin. However, since the conductor wires are lead out from the principal surface of the semiconductor chip, the resin should at least be of sufficient thickness to entirely cover and shield the wires.
Consequently, the purpose of the present invention is to minimize the warping of the package caused by thermal contraction in the aforementioned semiconductor device.